Removed unused class in callback.h
Removed unused class in callback.h

Change-Id: I2bd7423d12fd69d8ee1a36fafd8f77ec99370992
Reviewed-on: http://review.couchbase.org/43273
Reviewed-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

teligent-ru/ep-engine,jimwwalker/ep-engine,membase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,membase/ep-engine,membase/ep-engine,jimwwalker/ep-engine,hisundar/ep-engine,owendCB/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,sriganes/ep-engine,couchbase/ep-engine,daverigby/kv_engine,couchbase/ep-engine,daverigby/ep-engine,couchbase/ep-engine,hisundar/ep-engine,couchbase/ep-engine,daverigby/kv_engine,sriganes/ep-engine,jimwwalker/ep-engine,daverigby/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/kv_engine,membase/ep-engine,hisundar/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine